“A drink a day increases cancer risk” BBC news has said. Several other news sources have reported the findings of a large study that suggests that having just one alcoholic drink a day, whether it be wine, spirits or beer, “causes an extra 7,000 cancer cases – mostly breast cancer – in UK women each year”. Overall, the news says that alcohol is to blame for about 13% of breast, liver, rectum, mouth and throat cancers, and that 5,000 cases of breast cancer every year can be attributed to alcohol.

This study followed 1.3 million UK women, looking at their average alcohol consumption and whether they developed a number of different cancers. Increasing alcohol consumption by one drink per day was found to increase the overall risk of developing any form of cancer, plus several specific forms, including breast cancer. The researchers conclude that their results equate to alcohol causing at least 15 extra cancers per 1,000 women up to the age of 75.

There are several limitations that need to be taken into account when considering these findings. Current UK guidance for women is to avoid binge drinking and to consume no more than two to three units per day.

Where did the story come from?

Naomi E. Allen and colleagues of the Cancer Epidemiology Unit at the University of Oxford carried out this research, which was funded by Cancer Research UK, the UK Medical Research Council and the UK NHS Breast Screening Programme. The study was published in the peer-reviewed Journal of the National Cancer Institute.

What kind of scientific study was this?

This was a cohort study which aimed to look at the relationship between low-to-moderate levels of alcohol intake in women (typically fewer than three drinks per day or 21 drinks per week) and overall risk of cancer and of cancer at specific sites.

The researchers used data from the Million Women Study, which recruited 1.3 million middle-aged UK women attending breast screening between 1996 and 2001. The women completed a questionnaire asking them to report average weekly consumption of wine, beer and spirits, and recording socio-economic details. A follow-up survey was carried out three years later and this again checked alcohol consumption. All participants in the study were flagged on the NHS Central Register, and any new diagnoses or deaths from a wide range of cancers were identified.

When analysing their data the researchers excluded 42,408 women who had a registered cancer prior to recruitment plus 9,721 women who had information missing on their alcohol intake. This left 1.28 million women in the study.

The researchers classified these women into five groups according to their weekly intake of drinks that contained 10 grams of alcohol, the equivalent to one 125ml glass of wine, half a pint of beer or one measure of spirits with standard alcohol content. These five drink intake groups were none, one to two, three to six, seven to 14, or 15 or more drinks per week.

The researchers compared all other alcohol groups to those that took one to two drinks per week, as they considered that women drinking no alcohol may have stopped drinking due to ill health and therefore would not be a fair comparison group. In their analysis the researchers attempted to adjust for error in self-reported alcohol intake using a method known as regression dilution. This meant they effectively grouped women into the five groups according to consumption reported at baseline (the beginning of the study), but took their average alcohol consumption as that reported at follow-up.

Statistical methods were used to estimate risk of cancer at each site associated with various measures of alcohol intake. Adjustment was made for the possible confounding factors of socio-economic status, smoking, physical activity and use of the oral contraceptive pill or other hormonal therapies.

What were the results of the study?

The average age of women at recruitment into the study was 55 years. Most had low-to-moderate alcohol intake, with 24% reporting being non-drinkers, 29% drinking one to two drinks per week, 23% drinking three to six drinks, 19% drinking seven to 14 drinks, and only 5% drinking 15 or more drinks per week. Of those studied, 98% were drinking fewer than 21 alcoholic drinks per week.

Various factors were associated with drinking more, for example, affluence, doing strenuous exercise more frequently and being more likely to have used hormonal contraception or other therapies. Smoking increased with increasing alcohol intake.

Women were followed for cancer for an average of 7.2 years per woman, totalling 9.2 million person years. During follow-up, a total of 68,775 cancers were identified. Compared to those drinking one to two drinks per week, increasing consumption of alcohol by 10 grams per day was associated with a significant increased risk of any cancer, and specifically cancer of the mouth and throat, vocal cords, oesophagus, rectum, liver and breast. Exclusive drinking of wine (red or white) was associated with similar risks to other drinks.

The researchers found that the increased risk of cancers of the vocal cords, oesophagus and mouth and throat was limited to smokers, with no effect of alcohol in ex-smokers or never-smokers. Increasing consumption of alcohol was conversely linked to a decreased risk of thyroid cancer, cancer of the kidney and non-Hodgkin’s lymphoma.

Compared to those drinking one to two drinks per week, non-drinkers were also found to have significantly increased risk of cancer of the oesophagus, mouth and throat, stomach, liver, lung, cervix, lining of the uterus and kidney.

What interpretations did the researchers draw from these results?

The researchers conclude that low-to-moderate consumption of alcohol by women increases their risk of certain cancers. They say that for every extra drink consumed per day, this equates to 11 extra cases of breast cancer, one cancer of the mouth and throat, one cancer of the rectum and 0.7 extra cancers of the oesophagus, vocal cords and liver per 1,000 women. This results in an excess of 15 cancers per 1,000 women up to the age of 75 that are caused by alcohol.

What does the NHS Knowledge Service make of this study?

This is a strong study in that it has followed a large number of women and used reliable sources of data to provide information on cancer cases, and only analysed data on those women with complete alcohol intake information. The study has also demonstrated a trend towards increased risk of any cancer plus certain specific cancers.

Some important points to note about this research are:

The study has only assessed average alcohol intake at two times, three years apart. It is not possible to tell from this whether the responses represented a long-term established drinking pattern, or whether alcohol drinking levels varied over the women’s lives.

There was a potential for error caused by inaccurate reporting by women, for example incorrect estimation of number of drinks, different alcohol contents of drinks, or size of measure or wine glass used. The researchers did make a careful attempt to try to minimise the chance of this type of error.

Of the women in this cohort 53% were drinking fairly low levels of alcohol (either no alcohol, or one or two drinks per week, as in the comparison group used for calculations). There were fewer women with heavier alcohol intakes; therefore, it is difficult to make accurate cancer risk estimates for these women.

Although the researchers excluded women who had developed cancer prior to study recruitment, a woman’s cancer may have developed around the time of recruitment or during the three-year interval between then and the second questionnaire. This means it is possible that, in some cases, measures of alcohol consumption may indicate drinking at a time when women had cancer rather than in the time before their cancer began to develop. This makes it difficult to determine whether alcohol consumption caused the women’s cancer.

In their analysis the researchers adjusted for possible confounders such as smoking, exercise and socio-economic factors. However, it was not possible to adjust for all risk factors associated with each type of cancer. Smoking was still found to be the principle risk factor for cancers of the mouth, throat, larynx and oesophagus.

It is also worth noting that non-drinkers were found to have increased risks of certain cancers compared to women who drank one to two drinks per week. However, researchers say these findings are limited by the fact that they were not able to distinguish former drinkers (who may have stopped drinking for health reasons) from lifelong never-drinkers.

Current guidance for women is to avoid binge drinking and to consume no more than two to three units per day.